1. Field of the Invention
This invention concerns an oil recovery process and more specifically a surfactant flooding oil recovery process. Still more specifically, this invention is concerned with an oil recovery process using an aqueous fluid containing a water soluble alkylpolyalkoxyalkyl sulfonate or alkylarylpolyalkoxyalkyl sulfonate anionic surfactant and a water insoluble ethoxylated nonionic surfactant, which fluid is phase stable and suitable for flooding formations containing high concentrations of salt and/or divalent ions such as calcium and magnesium, which formations additionally have temperatures in the range of from about 70.degree. to about 300.degree. F.
2. Description of the Prior Art
Petroleum is normally recovered from subterranean formations in which it has accumulated, by penetrating the formation with one or more wells and pumping or permitting the petroleum to flow to the surface through these wells. Recovery of petroleum from formations is possible only if certain conditions exist in the formations. The petroleum should be present in the formation in an adequately high concentration, and there must be sufficient permeability or interconnected flow channels within the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluid. When the formation has natural energy present in the form of an underlying active water drive, or gas dissolved in the petroleum which can exert pressure to drive the petroleum to the producing well, or a high pressure gas cap above the petroleumsaturated portion of the formation, this natural energy is ordinarily utilized first to recover petroleum. Recovery of petroleum in this manner using natural energy is ordinarily referred to as primary recovery. When this natural energy source is depleted, or in the instance of those formations which do not contain sufficient natural energy initially to support primary recovery, some form of supplemental or enhanced recovery process must be applied to the formation in order to extract petroleum therefrom. Supplemental recovery is sometimes referred to as secondary or tertiary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment.
Waterflooding, which involves the injection of water into the subterranean, petroleum-containing formation for the purpose of displacing petroleum toward the producing well, is the most economical and widely practiced supplemental recovery method. Water does not displace petroleum efficiently, however, since water and oil are immiscible and also because the interfacial tension between water and oil is quite high. Persons skilled in the art of oil recovery have recognized this limitation of waterflooding and many additivies have been described in the literature for incorporation in the flooding water to reduce the interfacial tension between the injected water and the formation petroleum. U.S. Pat. No. 2,233,381 (1941) discloses the use of polyglycol ethers as surface active agents or surfactants to increase the capillary displacement efficiency of an aqueous flooding medium. U.S. Pat. No. 3,302,713 discloses the use of petroleum sulfonates prepared from specified boiling range fractions of petroleum feedstock as a surfactant in surfactant flooding supplemental oil recovery operations. Other surfactants which have been proposed for oil recovery operations include alkylpyridinium salts, alkyl sulfates, alkylaryl sulfates, ethoxylated alkyl or alkylaryl sulfates, alkyl sulfonates, alkylaryl sulfonates, and quaternary ammonium salts.
The above described surfactants are satisfactory in certain limited applications, particularly in formations containing water whose salinity and hardness, i.e. concentration of divalent ions including calcium and magnesium, is relatively low. For example, when the salinity of the formation water is less than about 30,000 parts per million and the concentration of divalent ions is less than about 200 to about 500 parts per million, petroleum sulfonates are very effective. U.S. Pat. Nos. 3,792,731; 3,811,504; 3,811,505; and 3,811,507 describe oil recovery processes employing specified mixtures of water soluble anionic and water soluble nonionic surfactants which exhibit satisfactory performance in petroleum formations containing water having high concentrations of divalent ions, i.e. in the concentration range of from about 500 to 18,000 parts per million calcium and magnesium.
U.S. Pat. No. 3,508,612 describes an oil recovery method using an aliphatic polyethoxy sulfate and an organic sulfonate. U.S. Pat. No. 3,888,308 describes the use of an aqueous solution of an alkyl or alkylpolyethoxy sulfate as a mobility buffer. U.S. Pat. No. 3,827,497 and 3,890,239 describe an oil recovery process and a fluid containing the organic sulfonate and an alkylpolyethoxy sulfonate. U.S. Pat. No. 3,977,471 describes an oil recovery process and a fluid employing an alkylarylpolyalkoxyalkyl sulfonate in single surfactant mode.
While these mixtures effectively increase the salinity concentration range of surfactant flooding, the higher salinities and higher concentration ranges of calcium and magnesium often involve the use of a sulfated, ethoxylated surfactant which is not stable at temperatures higher than about 125.degree. to 150.degree. F, because of the tendency for the sulfated ethoxylated compounds to hydrolyze. Unfortunately, many subterranean petroleum formations are known which contain water having high salinity and/or high concentrations of divalent ions such as calcium and/or magnesium, and additionally are at higher temperatures than 125.degree. to 150.degree. F.
It is desirable that the surfactant fluid viscosity by at least slightly greater than the crude oil present in the formation to ensure effective volumetric displacement. Hydrophilic polymeric materials such as polyacrylamides and polysaccharides are commonly used for forming a viscous fluid separate from the surfactant fluid to be injected after the surfactant fluid. Certain surfactants are incompatible with the common polymers, however, and there is a risk of phase instability in the use of these polymers in the same fluid as contains otherwise desirable surfactants.
In view of the foregoing discussion, it can be appreciated that there is a substantial, unfulfilled commercial need for a surfactant oil recovery method using a phase stable surfactant fluid, preferably a viscous fluid which can be injected into formations whose temperatures are in excess of 125.degree. F., which formations also contain water having salinities higher than about 30,000 parts per million total dissolved solids, and/or conentrations of divalent ions greater than about 2,000 parts per million.